The present invention relates generally to the field of DC to DC converters used with non-linear devices such as light emitting diodes (LED) in back lighting for liquid crystal displays (LCD).
Many crystal displays (LCDs) take advantage of light emitting diode (LED) back lighting technology. The LEDs are used to provide a back lighting source for the LCDs so that the displays may be more efficiently viewed. However, illuminosity of LEDs is sensitive to current fluctuations and is directly dependant upon the current flowing through the LEDs. Therefore, back lighting LED circuitry for LCDs must regulate the current flow through the LED to ensure a constant current during all operating conditions. thus providing a constant lighting source for the LCDs.
LEDs utilized for back lighting purposes are typically aligned in an array. FIG. 1 illustrates a block diagram of a typical circuit utilizing a LED array in the prior art. LED 101 is placed in series with a predetermined number of equivalent LED 101s. This multiple LED arrangement provides a stack 105 of four LEDs aligned in series. Those skilled in the art will understand the stack is not limited to four LEDs and that more or less than four LEDs may be used to achieve similar results.
Subsequent stacks 106 and 107 are placed in parallel with stack 105. Stacks 106 and 107 each have the same LED arrangement as stack 105, each stack consisting of the same number and type of LEDs. The parallel arrangement of stacks 105, 106 and 107 provides an LED array 110 for the back lighting of the LCD (not shown).
Power requirements of LEDs encourage the LEDs to be stacked in series, with each stack then placed in parallel with other stacks as shown in FIG. 1 in circuit 110. As diodes, LEDs require some voltage to forward bias the LED and permit proper operation of the light emitting aspect of the diode. Typical LEDs might require as much as 1.2 volts or more to forward bias the diode depending on the diode used. Thus, when four LEDs are connected in series as a LED stack 4.8 volts may be required to forward bias the stack. Additional circuit losses might force the voltage requirements of the stack to be as high as 8 volts to forward bias the LED stack. This level typically exceeds the standard voltage used in the display circuitry requiring a step-up or boosting circuitry to provide the additional voltage. Many in the art have solved this problem by using a DC-DC converter to step-up or boost the voltage available to the diode stacks. Thus, voltage source 125 is input to a DC-DC boosting converter 120 which typically provides a boosted voltage across the LED array 110. Current regulating device 130 senses the current across resistor 135 and regulates the current through LED array 110 in order to provide a constant current through display 110. Those skilled in the art will recognize that the arrangement of stacks 105, 106 and 107 along with the current limiting device 130 provide a constant voltage and current source across LED array 110, thus providing a constant illuminosity output of display 110. In this way, those skilled in the art will recognize that each LED will receive the same current flow as every other LED in the array 110, ensuring a constant luminosity across the LED array 110 for any given current flow.
FIG. 2 illustrates a traditional DC-DC converter 120 utilized in a boosting circuit to provide the necessary voltage needed for back lighting LEDs. Shown in FIG. 2 is a DC/DC converter control chip 203. DC/DC converter control chip 203 is a generic, standard DC/DC converter control chip such as Advance Micro Device""s ADP 1110. Various inputs are needed for DC/DC converter control chip 203 to operate in its normal mode. Pin 1235 is supplied with VIN 250 which may correspond to the bus voltage within the display. Additionally, DC/DC converter control chip 203 receives a feedback signal (FB) 245 at pin 3. The feedback voltage samples the voltage across a typical voltage divider resistor circuitry. The voltage divider circuitry consists of resistors 210 and 220. The DC-DC converter 120 compares the feedback voltage with a reference voltage determined by DC/DC converter control chip 203, internal to the DC-DC converter 120. The reference voltage source commonly used in DC-DC voltage converters varies from 0.22 volts to 1.245 volts or more.
Switch (SW) 230 at pin 2 regulates the current through inductor 202. By regulating the current through inductor 202, those skilled in the art will recognize that, in combination with rectifier 205 and capacitor 225, inductor 202 will operate to provide a boosted, VBUS 260 which is greater than VIN. Using a resistor divider feedback circuitry, a constant VBUS 260 is maintained. Thus, a separate and distinct current regulating circuit as shown in FIG. 1, at 130, is needed to provide a constant current through the LEDs.
FIG. 3 provides an example of typical current regulating circuitry 130 utilized in a back light LED circuit in prior art circuits. In FIG. 3, a certain VBUS 260 is provided from DC-DC converter 120 as a constant voltage source to the current regulating circuit 130. Four LEDs 101 are placed in series to create LED stack 105 as described above in FIG. 1. Transistor 320 and low ohmic resistor R1 135 are placed in series with LED stack 105. Operational amplifier 330 senses the voltage across resistor 135 and compares the voltage across resistor 135 with a reference voltage 340 maintained by xenor diode 335 and resistors 336 and 338 which are aligned in a typical resistor divider network. When current flow through LED stack 105 varies from a predetermined range, those skilled in the art will recognize that Operational amplifier 330 will sense the current divergence by comparing the two voltages. If the current flow through the LED stack 105 is less than the predetermined value, the circuitry will bias transistor 320 on or off accordingly to maintain the proper current flow through LED stack 105. It is important to note that in the prior art example shown in FIG. 3, a constant voltage source 260 is provided as a VBUS as described above. Thus, the current regulating circuitry 130 directly affects the current flow through LED stack 105 without affecting the voltage 260.
Because LEDs tend to be non-linear resistive devices, a stack of LEDs is unable to be used in a voltage divider network in order to regulate the current through the stack. Stated another way, LED stack 105 cannot replace resistor 135, simply because of LED""s non-linear characteristics do not allow for a predictive voltage to occur across a LED or LED stack. Thus, the voltage across a LED or LED stack cannot be used as the feedback voltage for a constant current output. Hence, in prior art solutions, a DC-DC converter 120 alone is unable to regulate the current through the LED back light display 110 to maintain a constant current through the LEDs. Thus, prior art solutions incorporated a separate current sensing circuit 130 electrically coupled to the DC-DC converter to regulate the current through the LED stack to within the proper limits to maintained the desired luminosity as described above. Using two separate circuits to provide the necessary voltage and current flow through the LED array 110 is inefficient because of the additional circuitry needed.
Thus, a need in the art exists for a simplified DC-DC converter/current regulating device that, all in one, provides the proper voltage range, while maintaining the required current needed to provide a constant luminescence in the back light LED art.
Accordingly, provided is a DC-DC voltage converting means that takes the input voltage and either boosts or bucks the voltage to an output voltage, the output voltage being placed across a LED array and low ohmic resistor, such that the voltage across the low ohmic resistor may be utilized as a feedback signal to control the output current, thus providing a constant current through the LED array. If the LED array demands it, an op amp is used to properly amplify the voltage sampled across the low ohmic resistor such that the output voltage from the op amp is within the specifications of the DC-DC converting circuitry.
Additionally, provided is a circuit comprising a DC-DC converting means for providing a boosted or bucked output voltage, the output voltage being provided to an electrical device connected in series with a current sensing means which provides a voltage feedback to the DC-DC converter, the voltage feedback being used to regulate the output voltage such that the current through the electrical device is constant within the data processing system.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.